Starting from version 0.24 you could view the amount of forge points that individual player contributed to each level of the great buildings (in version 0.23 you could view only the contributions to the current level).
In the case when you don't have any forge points available if you start navigating between the various levels of a friend's great building when you return to the current level's page the "Use 2 Forge Points" button will be displayed and it will be active. The "Use 1 Forge Point" button next to it will still be inactive.
If you click on the active "Use 2 Forge Points" button nothing happens at the first sight. But if you to the list of great buildings where you are contributing in the town hall's "News" tab you will noticed that you are listed as a contributor to that great building. If there aren't too many contributors you might even get rewarded for the position occupied in the contributors list: you'll get forge points, medals and blueprint pieces (I am not sure of this as the current level is not finished yet!).
See attached screenshots.